According to the World Health Organization, health effects are most pronounced in pH extremes. Drinking water with an elevated pH above 11 can cause skin, eye and mucous membrane irritation. On the opposite end of the scale, pH values below 4 also cause irritation due to the corrosive effects of low pH levels. WHO warns that extreme pH levels can worsen existing skin conditions.
The normal range for pH in surface water systems is 6.5 to 8.5 and for groundwater systems 6 to 8.5. Alkalinity is a measure of the capacity of the water to resists a change in pH that would tend to make the water more acidic. ... In general, a water with a low pH (< 6.5) could be acidic, soft, and corrosive. If your water is alkaline, you get healthy minerals in it, if your water is acidic, you can absorb toxins like mercury from it.
It is important to test your water every five years at a minimum.